Meeting report: Is the Earth special?

If very special preconditions are necessary for the eventual evolution of observers on a planet then the Earth must be an oddball. If true, this “anthropic selection” concept is as important for a clear understanding of our planet’s properties as the concept of plate tectonics! But is it true? On December 9, 2011 more than ninety geologists, biologists and astronomers gathered at an RAS discussion on “Is the Earth special?” to debate this issue at a meeting organized by the Astrobiology Society of Britain (an affiliate organization of the RAS).

The meeting was organized into three sessions (on astrophysics, on planetary science and on biology) but began with a keynote lecture from Jim Kasting (Penn State) explaining why he believes the “Rare-Earth” hypothesis to be too pessimistic. The key parameter, in his view, is the fraction of stars with planets in the habitable zone (HZ) and, since the climate stabilizing effect of silicate weathering produces a relatively wide HZ, up to a third of all stars should have planets at the appropriate distance. Nevertheless, as Professor Kasting pointed out, there are other important constraints such as planet mass, stellar mass and the question of whether planetary formation mechanisms typically deliver the necessary proportion of volatile compounds (especially CO2 and H2O) to planets in the HZ.

Andrew Liddle (University of Sussex) then outlined the cosmological context of the discussion. Anthropic selection arguments are more fully developed in cosmology than they are at the planetary level and have recently been given increased credibility by the fact that multiple-universes are a natural consequence of inflationary models for the early universe. Inflationary models, in turn, are spectacularly effective at predicting the power spectrum of the cosmic microwave background and are therefore widely accepted by cosmologists. Crucially, inflation predicts that our Universe is much larger than the small part we can see and thus, even if Earth-like planets are vanishingly rare, they are all but inevitable somewhere in the cosmos.

The meeting then moved onto planetary formation theory. Richard Nelson (QMUL) provided an up to date survey of N-body simulations of planet formation and, in particular, how migration of both low and high mass planets affects the formation of terrestrial worlds. Migration of giant planets from beyond the snow-line through the terrestrial zone was shown to endow terrestrial planets with abundant water and other volatiles, such that “ocean worlds” are very likely to form. To date, however, no models exist that self-consistently treat the formation and migration of terrestrial and giant planets, so theoretical explanations about the formation of the solar system, and predictions about the nature of alien worlds remain rather tentative.

The second session of the morning began with back to back talks on “Where did it all go wrong for Mars?” (Monica Grady, OU) and “Venus, the Earth that never was” (Richard Ghail, IC) which contrasted the Earth’s evolution with those of our nearest neighbours. Both talks emphasised that Venus, Earth and Mars were made from similar starting materials by similar processes but that both our neighbours became permanently unsuitable for complex life early in their history. In the case of Mars, the key factor seems to be its relatively small size and hence rapid cooling which resulted in the loss of plate tectonics by 3.5 billion years ago. Venus, on the other hand, although much closer in size to Earth is significantly less dense, indicating a smaller core unable to generate a magnetic field. Being closer to the Sun it perhaps accreted less water but its warmer temperature and lack of magnetic field allowed the solar wind and/or neutral-hydrogen thermal-escape to strip away any water it did have. The lesson is that neither Mars nor Venus would have become inhabitable had they happened to form at the location of the Earth.

The final three talks of the morning looked at what little data we have and what we can infer from it. Andrew Watson (UEA) began with the observation that the emergence of intelligent life has taken 80% of the time available between the origin of life (around 4 billion years ago) and a time (roughly 1 billion years in the future) when the slowly-evolving Sun will become too warm for life on Earth. This coincidence of time scales can be quantitatively explained by assuming that 4 extremely unlikely but critical steps were required for the evolution of intelligence. Each of the steps has very low probability of occurrence within the habitable lifetime of a planet and hence the implication is that intelligent life (which requires all 4 steps) is vanishingly rare. Habitable is not the same as inhabited!

Ian Crawford’s (Birkbeck) talk could accurately be described as being based upon the absence of data! Fermi’s Paradox (Where are they?) stems from the observation that there is no evidence of any outside interference in the solar system at any point in its 4.5 Gy history. If advanced civilizations are common in our Galaxy then a simple mathematical model shows that at least one of them should have colonized every suitable planet on a time-scale very much shorter than the Galaxy’s lifetime. There are many possible explanations for why this has not occurred but almost all require implausible assumptions about the consistency of alien psychology (e.g. that advanced civilizations always wipe themselves out, or that they subscribe to the same set of ethical codes regarding interference with more primitive life-forms). The only explanation not ruled out is that advanced civilizations are rare. Crawford argued that this is unlikely to be due to an absence of suitable planets, however, and more likely to be a consequence of the complexities of evolutionary biology.

Dave Waltham (RHUL) then discussed the limited astronomical data currently available. In particular: 95% of all stars in our neighbourhood are smaller than the Sun; 96% of large exoplanet orbits (i.e. > 0.1 AU) are less circular than the Earth’s and 95% of our Galaxy’s stars are closer to the centre of the galaxy than we are. This data, together with Bayes’ Theorem and the “Principle of Mediocrity” (i.e. that the Earth should be a typical inhabited planet) then allows the habitable-range for each of these properties to be estimated. The best estimates are that 10% of stars have masses in the habitable range, 7.4% of planets have orbital eccentricities in the habitable range and 11% of stars are within the galactic habitable zone. Unfortunately, the current data is very sparse and biased and the formal uncertainties in the results are very large. Hence, at present, it is hard to draw more than tentative conclusions but the Bayesian-analysis technique should come into its own as better data becomes available.

Most of the morning’s talks therefore produced a rather gloomy assessment of the prevalence of habitable worlds but our afternoon keynote lecture from Helmut Lammer (Austrian Academy of Sciences) gave a more upbeat approach by considering alternative habitats such as icy-moons and ice-giants which have migrated into the habitable zone. Depending on the formation age of the planet, its mass and size, as well as the life-time on the EUV-saturated early phase of its host star, many terrestrial planets may not get rid of their proto-atmospheres and could end as water worlds with CO2 atmospheres and hydrogen or oxygen-rich upper atmospheres. If an atmosphere of a terrestrial planet evolves to an N2-rich atmosphere too early in its lifetime the atmosphere may be lost by various escape processes. This talk demonstrated that the route to Earth-like habitability is easily disrupted by early stellar-activity, differences in volatile content and by the role of impacts. As a result, non-Earth-like habitats may be more common than Earth-like ones!

The following talk continued the theme of looking at alternatives to Earth by considering the question of whether Earth-like biospheres, characterized by complex organisms living upon the surface of a planet, are the exception rather than rule (Sean McMahon, Aberdeen). The deep biosphere is a relatively recent discovery on Earth and it forms a much more stable and predictable environment than the near-surface since “surfaces are extreme environments” which experience the effects of cold, radiation and impacts. Moreover, almost all the liquid water belonging to planets and moons is likely to occur well outside of traditional circumstellar HZs, in geothermally or tidally heated subsurface environments. Although such environments may commonly meet other criteria for habitability, it is not clear whether they can host independent origins of life. If they can, then subsurface biospheres are probably far more common in the Universe than surface-dominated ones.

In our penultimate talk of the day Nick Lane (UCL) returned to the earlier theme of whether there are intrinsically unlikely steps in the evolution of complex organisms. Nick demonstrated that redox and proton-gradients across cell-boundaries are “as universal as the genetic code” and essential for many fundamental biochemical processes such as respiration and photosynthesis. This suggests that the origin of life might be favoured on any wet, rocky planet with a CO2 atmosphere, as these conditions promote serpentinization and the formation of vent systems with remarkable congruence to living cells, including redox and protein gradients across membranes! However, the central importance of the gradients substantially restricts the largest practical size that a cell can grow to and explains why the majority of Earth’s organisms remain microscopic. The ability to grow a larger, multi-celled organism required endosymbiosis between prokaryotes which give rise to the eukaryotic cell and this may have been an unlikely, once-only, event in the history of our planet.

The discussion was drawn to a close by Lewis Dartnell’s (UCL) presentation on extremophiles. These organisms demonstrate that life is highly adaptable. Some of the stranger organisms discussed were were grylloblatid insects that live on Arctic ice fields at subzero temperatures (and would die even with the warmth of your hand), bacteria which metabolize uranium and animals from 3km below the surface of the Mediterranean which spend their entire lifecycle without using oxygen. The environmental envelope occupied by these organisms overlaps with environments met on Mars, in the clouds of Venus and deep beneath the surface of Europa. Life can adapt to a wide range of environments and the main constraint on its distribution is therefore likely to be due to how hard it is for life to arise in the first place.

The general thrust of the talks at this discussion meeting was that, whilst simple life may well be widespread, there are many possible barriers to the development of intelligent observers in the Universe. Suitable habitats may be rare and, to make matters worse there may be several critical and unlikely steps in the development of sentient life. However, given the current paucity of hard data, we cannot know how rare life is in general, and intelligent life is in particular, unless we keep looking.